Density tuning of one-dimensional electron gas
in a T-shaped quantum wire
Toshiyuki Ihara, Masahiro Yoshita, Hidefumi Akiyama
Institute for Solid State Physics, University of Tokyo, and CREST, JST, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
Loren N. Pfeiffer, Ken W. West
Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974
E-mail address: ihara@issp.u-tokyo.ac.jp
Abstract: Variable-density one-dimensional electron gas was realized in a T-shaped quantum wire with
an FET gate. We achieved photoluminescence-excitation measurements on a single wire and observed
spectral evolution from degenerate to non-degenerate one-dimensional electron gas.
©2007 Optical Society of America
OCIS codes: (160.6000) Semiconductors, including MQW; (300.6470) Spectroscopy, semiconductors
T-shaped quantum wire is one of semiconductor quantum structures for investigation of one-dimensional (1D)
systems [1]. By fabrication of an FET gate and n-type modulation doping, variable-density 1D electron gas can be
formed [2]. In this work, we studied low-temperature photoluminescence-excitation (PLE) spectra in conjunction
with PL spectra in an n-type doped T-shaped quantum wire with an FET gate.
Figure 1 shows the sample structure. Cleaved-edge overgrowth with molecular beam epitaxy and growth interrupt
annealing were used to fabricate the single quantum wire sample with a 14nm x 6nm cross-sectional size [2]. An
FET gate layer was placed at the top of the (110) surface to tune 1D electron density of the wire. Micro-PL and PLE
measurements on the wire were performed at 5K with excitation from cw titanium-sapphire laser with a 1m spot
size.
Figure 2(a) shows PL (dotted line) and PLE (solid line) spectra of the 1D wire at gate voltage (Vg) of 0.7 V. At this
voltage, a dense electron gas is accumulated in the wire so that optical spectra show typical structures of
band-to-band transitions. The PL spectrum is dominated by a large peak (BE) at 1.565eV, which we assigned to
band-edge emission of 1D electron gas. The PLE spectrum shows an onset (FE) at 1.575eV with a low-energy tail,
which we assigned to Fermi-edge absorption onset. Dashed line shows theoretical emission and absorption spectra
fitted to the experimental data. We used free-particle model without many-body Coulomb interaction. The 1D
electron density was estimated to be 6 x 105 cm-1, which corresponds to the Fermi energy of 5 meV. Thus, at low
temperature (5K), the 1D electron gas should be degenerate.
Figure 2(b) shows PL and PLE spectra at Vg = 0.0 V. At this voltage, the electron gas is completely depleted so that
the spectra behave as if the system is non-doped. Both PL and PLE spectra show a strong peak (X) at 1.5685 eV,
which we assigned to neutral excitons (X). The full width at half maximum (FWHM) of the X peak was 0.9 meV,
and the stokes shift between PL and PLE peak is less than 0.2 meV. These small values indicate a high quality of our
sample. Note that the PLE spectrum at 0.0 V is almost the same as that of non-doped quantum wires [3]. The
absorption onset at 1.579 eV is assigned to continuum onset and the exciton binding energy is estimated to 13 meV.
Fig. 1: Schematic structure of n-type doped T-shaped quantum wire with a gate to tune electron
density. One-dimensional electron gas is formed at the cross-sectional area of 14nm stem well and
6nm arm well.
Fig. 2: Normalized PL (dotted lines) and PLE (solid lines) for 1D quantum wire at the gate
voltage of (a) 0.7V and (b) 0.0V. The structures at BE, FE and X are assigned to the band edge,
Fermi edge and neutral excitons, respectively. Dashed lines are theoretical emission and absorption
spectra fitted to the experimental data with free-particle model calculations.
Fig. 3: Normalized PL (dotted lines) and PLE (solid lines) for 2D arm well at the gate voltage of
(a) 0.8 V and (b) 0.2 V.
As shown above, we observed spectral evolution from the band-to-band structures at Vg = 0.7 V to the excitonic
peaks at Vg = 0.0 V. This represents realization of a variable-density 1D electron gas from 6x105 cm-1 to almost zero
in our T-shaped quantum wire with an FET gate. By tuning the electron density and the temperature, we can control
the degeneracy of the 1D electron gas.
It is also notable that we achieved PLE measurement on a single quantum wire of extremely small volume. This
was made possible by development of highly sensitive PLE measurement system, where the direction of PL
detection was perpendicular to the laser excitation and their polarizations were orthogonal to each other. Thanks to
this geometry, we could eliminate the intense laser scattering and improve signal-to-noise ratio of PLE spectra.
Setting the target position of the measurement to the arm well, as indicated in the inset of Fig. 3, we also measured
PL and PLE spectra of 2D electron systems. Figure 3(a) shows PL and PLE spectra measured for the 2D arm well at
Vg = 0.8 V. We observed a PL peak at 1.581 eV, and two PLE peaks at 1.585 eV and 1.593 eV. These structures are
analogous to Hawrylak’s model [4]. At Vg = 0.2 V, the 2D electron gas is depleted so that a strong excitonic peak
appears at 1.5835 eV in both PL and PLE spectra, as shown in Fig. 3(b). The absorption onset at 1.591 eV is assigned
to 2D continuum state.
[1] H. Akiyama, M. Yoshita, L. N. Pfeiffer and K. W. West, ”One-dimensional excitonic states and lasing in highly uniform quantum wires
formed by cleaved-edge overgrowth with growth-interrupt annealing”, J. Phys.: Condens. Matter 16, S3549 (2004).
[2] H. Akiyama, L. N. Pfeiffer, A. Pinczuk, K. W. West, and M. Yoshita, “Observation of large many-body Coulomb interaction effects in a
doped quantum wire”, Solid State Commun. 122, 169 (2002).
[3] H. Itoh, Y. Hayamizu, M. Yoshita, H. Akiyama, L. N. Pfeiffer, K. W. West, M. H. Szymanska, and P. B. Littlewood, “Polarization-dependent
photoluminescence-excitation spectra of one-dimensional exciton and continuum states in T-shaped quantum wires”, App. Phys. Lett. 83, 2043
(2003).
[4] P. Hawrylak, “Optical properties of a two-dimensional electron gas: Evolution of spectra from excitons to Fermi-edge singularities”, Phys.
Rev. B 44, 3821 (1991).